FIG. 12 shows an electron beam apparatus of image projecting type according to a prior art, in which a primary electron beam is radiated onto a surface of a sample, and a secondary electron beam emitted from the surface of the sample is image-projected onto a multi-channel plate (MCP) so as to illuminate a fluorescence screen by multiplied electrons, so that a secondary electron image signal may be obtained by a plurality of detecting elements. As shown in FIG. 12, reference numeral 90 designates a TDI camera disposed in an atmospheric environment side and reference numeral 91 designates a MCP/FOP assembly disposed within a chamber in vacuum condition. This assembly 91 is equipped with a TDI (Time Delay and Integration) sensor 92, which is a suitable detecting element for the electron beam apparatus of image projecting type. The TDI sensor 92 disposed in the vacuum side is electrically connected with the TDI camera 90 disposed in the atmosphere side via a feed-through device mounted to a flange 93.
FIG. 13 and FIG. 14 show a feed-through device 94 mounted to the flange 93. As shown in FIGS. 13 and 14, the feed-through device 94 mainly comprises a plurality of pins 96 disposed in the TDI camera 90 side and attached to the flange 93, a plurality of pins 95 disposed in the TDI sensor 92 side, and a plurality of sockets 97 for connecting the pins 96 with the pins 95. Mating the pin 95 to the socket 97 as well as the pin 96 to the socket 97 may establish an electrical connection between the TDI sensor 92 disposed in the vacuum side and the TDI camera 90 disposed in the atmosphere side.
In the prior art feed-through device 94 shown in FIGS. 13 and 14, if a number of pins used for the connection is not so much, even a small connecting force (resistance force) should be sufficient for mating the pins with the sockets, but if a large number of pins is used for the connection, a great connecting force should be necessary, which sometimes causes a problem that a semiconductor sensor to be connected by the feed-through device could be broken by this great connecting force. For example, assuming that the force of 1 kg is required for the coupling of one pin and 100 pieces of pins are used for the connection, the total force of 100 kg may be required for the complete connection, which could sometimes result in a breakage of the semiconductor sensor or the like if any forced coupling would be applied.
In contrast, when a small number of pins is used for the connection, although there may occur no breakage in the semiconductor device owing to rather small connecting force, there has been another problem that, upon inspecting a wafer having an extended surface, the time required for the inspection should be longer due to such a small number of pins for connection and also a highly accurate and reliable inspection could not be carried out in an effective manner.
As for an exemplary TDI sensor 92, such a specification has been employed that includes, for example, a pixel size of 16 μm, a horizontal resolution of 2048 pixels, a number of integration stages of 512, a number of taps (a number of signal terminals of the TDI sensor) of 32, a line rate of 250 kHz, and a data rate of 25 MHz×32 (taps)=800 MHz. In this configuration, a plurality of line imaging devices, each comprising 2048 pixels of CCD imaging device arranged in a longitudinal direction, are arranged laterally in 512 lines so as to form a rectangular shape.
The TDI sensor is oriented such that the lateral direction thereof is arranged in parallel with the Y-direction corresponding to a scanning direction on a sample while the longitudinal direction thereof is arranged in parallel with the X-direction. That is, a stage transfers the sample successively along the lateral direction of the TDI sensor. At that time, a linear region on the sample covered by 2048 pixels, whose image has been picked up by one of the line imaging devices in the TDI sensor, maybe subjected to the following adjacent image sensors sequentially along the scanning direction for the image thereof to be picked-up. When the stage movement is controlled to be in synchronism with the signal outputs from the line imaging device, then total 512 pieces of time delayed signals obtained through one TDI sensor by sequentially delaying the output signals from 512 pieces of line imaging devices should correspond to the image data representing the one linear region on the sample covered by the 2048 pixels. Then, the one TDI sensor adds up those 512 pieces of time delayed signals and then output the result. This adding operation can offset noise components in respective line imaging devices and thereby improve a S/N ratio of the image data signal considerably. Further, when the delay time for the output signals from the 512 line imaging devices were shifted, then similarly, total of 512 pieces of time delayed signals can be obtained also for adjacent another linear region on the sample. The one TDI sensor adds up and thus outputs those signals for each linear region in the sequential manner, and ultimately an image signal representing the entire region on the sample to be inspected can be obtained.
In the TDI sensor of the above example, an effective region will be defined as 32.768×8.192 mm, which means that when this TDI sensor is used in a defect inspection apparatus, a field of view at the image projecting position should be 33.776 mm, which is a diagonal line length of the sensor effective region. Due to this, the image projecting optical system must be designed so as to control an aberration to be below a certain value over the entire field of view at this image projecting position. The length of a lens barrel satisfying this specification could be as long as 1 m. Accordingly, a certain scale of anti-vibration system is required, which could put some restrictions on a size of the apparatus. Since a law of similitude can be applied to the optical system using exclusively an electrostatic field, if the field of view may be designed to be narrower then the length of the lens barrel can be also made shorter in proportion therewith.
In addition, if a distortion in the projected map is greater, then an image mismatch between the images at an edge area and at a central area in the integration of the TDI sensor may also become greater, and such an integrated effect may appear as an image out of focus. To handle with this problem, the design specification requires the distortion to be reduced, for example, to a degree of 1.6 μm, which is equivalent to 1/10 pixel, over the entire field of view, and this requirement leads to more complicated optical mechanism and accordingly much longer lens barrel. From the fact that the distortion is increased in proportion with a size of the field of view by cube, the smaller the field of view is, the smaller the distortion and thus the mismatch of images between the edge area and the central area in the integration will be, and also the smaller field of view allows a larger distortion and thereby the optical mechanism can be simplified and the lens barrel can be made shorter.